Examples of conventional laser welding quality evaluation methods are described in Patent Document 1 and Patent Document 2.
The laser welding quality evaluation method described in Patent Document 1 separates by wavelength the light produced from the laser irradiation position into the reflected laser light and the vapor light such that the welding laser light and the optical axis are on the same axis. Then the welding quality evaluation method conducts measurement and thus evaluates a welding quality.
The laser welding quality evaluation method described in Patent Document 2 interprets the reflected laser light as an image and then conducts evaluation of the quality of the welding.
Furthermore, on the laser processing apparatus that conducts laser welding, a protective glass is conventionally provided to protect the optical system of the laser from sputter (flying bits of melted metal) and fumes (smoky gas) produced at the time of laser welding. Patent Documents 3 and 4 describe methods of monitoring the status of the protective glass.    Patent Document 1: Japanese Patent Application Publication No. JP-A-2000-42769    Patent Document 2: Japanese Patent Application Publication No. JP-A-2006-043741    Patent Document 3: Japanese Patent Application Publication No. JP-A-4-118193    Patent Document 4: Japanese Patent Application Publication No. JP-A-2002-361452